Field of the Invention
The present invention relates to an optical system and an image pickup apparatus including the same, which are suitable for, for example, an image pickup optical system to be used for an image pickup apparatus, e.g., a digital still camera, a digital video camera, a television (TV) camera, or a monitoring camera.
Description of the Related Art
Optical systems to be used for image pickup apparatus are demanded to have a small total length of a zoom lens and small weight as a whole. Here, the “total length of a zoom lens” refers to a length obtained by adding a value of air-equivalent back focus to a distance from a first lens surface to a final lens surface of the zoom lens. In general, telephoto lenses (telephoto optical systems) with a large aperture ratio tend to have a large total length of the zoom lens and a large total weight of the zoom lens.
In general, in optical systems that are small in size and weight, chromatic aberrations such as an axial chromatic aberration and a lateral chromatic aberration tend to be generated by a large amount, leading to deterioration in optical characteristics. In Japanese Patent Application Laid-Open No. H11-119092, there is described an optical system that employs a lens made of a material having an extraordinary partial dispersion characteristic in order to reduce chromatic aberrations.
Specifically, in Japanese Patent Application Laid-Open No. H11-119092, there is described a telephoto optical system that employs, in order to reduce chromatic aberrations, a positive lens made of a low dispersion optical material having an extraordinary partial dispersion characteristic, e.g., fluorite, and a negative lens made of a high dispersion optical material. In telephoto lenses, chromatic aberrations are generally corrected with lenses on an object side that axial paraxial rays and pupil paraxial rays enter at relatively high incident heights. Here, the “axial paraxial ray” is a paraxial ray that is light entering an optical system in parallel with the optical axis of the optical system at a height 1 from the optical axis, with a focal length of the entire optical system being normalized to be 1. Further, the “pupil paraxial ray” is a paraxial ray that is one of light rays entering the optical system at an angle of 45° to the optical axis, and passes through an intersection between an entrance pupil of the optical system and the optical axis, with the focal length of the entire optical system being normalized to be 1.
In Japanese Patent Application Laid-Open No. 2011-81064, there is described an optical system that employs, in addition to a lens made of a low dispersion optical material having an extraordinary partial dispersion characteristic, a lens made of a high dispersion optical material having an extraordinary partial dispersion characteristic in order to correct chromatic aberrations, and reduce the entire optical system in weight.
Further, in Japanese Patent Application Laid-Open No. 2012-193065, there is disclosed a high dispersion optical material having a high refractive index and a low extraordinary partial dispersion characteristic. Hitherto, high dispersion materials having a high refractive index tend to have a strong extraordinary partial dispersion characteristic. Using a high dispersion material having a high refractive index as a material of a negative lens arranged on a relatively object side in a telephoto lens is effective for primary achromatization, but has a difficulty in reducing a secondary spectrum effectively. Meanwhile, the material disclosed in Japanese Patent Application Laid-Open No. 2012-193065 has a weak extraordinary partial dispersion characteristic although the material is a high dispersion material having a high refractive index. Thus, chromatic aberrations can be satisfactorily corrected with the use of lenses made of such a material.
In telephoto optical systems, as a focal length of the telephoto optical system is increased, lenses arranged on an object side in the optical system are increased in size and weight, leading to an increase in generation amount of chromatic aberrations. In order to obtain a telephoto optical system with high optical characteristics, which is small in size and weight as a whole and is configured to satisfactorily correct chromatic aberrations, it is important to appropriately set the structure of the lenses arranged on the object side in the optical system.